Wireless local area networks (WLANs) typically transmit via radio or infrared frequencies to connect data devices. In a WLAN, the wireless communication devices are often mobile, moving around more or less freely within the networked area. WLANs combine with infrastructure networks systems that can be connected to the Internet, thereby providing communication over long distances.
WLANs link portable and wireless computer devices, also called mobile stations or terminals, to a wired network via a plurality of fixed access points (APs), also called base stations. Allowing WLAN devices to communicate with the infrastructure network, access points provide for wireless communications within respective cells and are typically spaced throughout a designated networked area. The access points facilitate communications among a networked set of 802.11-compliant devices called a basic service set (BSS), as well as communications with other BSSs and wired devices in or connected to wired infrastructure network systems.
WLANs have been used in proprietary business applications such as order entry, shipping, receiving, package tracking, inventory, price-markdown verification, and portable point of sale. Such systems may have an operator carrying a handheld computer device that communicates with a server via one or more access points such as a wireless bridge or router, each access point interacting with the server to create a wireless cell.
The most common WLAN technologies are described in the Institute of Electrical and Electronics Engineer's IEEE 802.11 family of industry specifications, which include two physical-layer standards: 802.11b operating at 2.4 GHz and delivering up to 11 Mbps at 250 feet maximum; and 802.11a operating at 5 GHz and delivering up to 54 Mbps at 150 feet maximum. A third standard, 802.11g, which provides the speeds of 802.11a at the distances of 802.11b, is scheduled for finalization in late 2003. IEEE 802.11 specifies Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) for devices operating within an 802.11 wireless network. Informative material may be found in IEEE Std. 802.11-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, reference number ISO/IEC 8802-11:1999, ANSI/IEEE Std. 802.11, 1999 edition, 1999.
When a wireless device moves around a WLAN, it may need to change its present association from one access point to another if the reception level of the presently associated access point becomes too low, and a different access point provides a higher reception level. The procedure, known as roaming, allows a WLAN device to switch association among access points, a change that is generally based on the relative reception levels of the access points involved. Roaming procedures may be based on selected configuration settings for the access points (APs), such as density levels of cell sizes that influence their defer, carrier-detect, and cell-search behaviors. The term “roaming” as used here in association with WLAN systems, has a different meaning than in cellular telephony systems. In cellular telephony systems the term roaming refers to a subscriber unit traveling out of a home network service area and into a non-home network service area, where the operators of the home and non-home networks have an agreement to provide communication service to each other's subscribers. In WLAN systems, roaming refers to changing the presently associated access point providing wireless network service, which is more analogous to “hand off” between serving cells in a cellular telephony systems.
Within the wireless networks, wireless communications are generally managed according to an operating protocol that requires ongoing wireless activity to monitor the roaming of WLAN devices and to synchronize radio timing between these portable devices and access points. This ongoing activity contributes to the draining of power from battery-powered WLAN devices. Synchronization of radio timing becomes especially critical in the management of wireless communications, and more efficient scheduling of future coordinated activities provides better power-saving strategies since it allows the mobile WLAN device to conserve power between times when it must receive or transmit.
Before a WLAN device can communicate with other devices in a given WLAN, it must first locate access points. The medium access control (MAC) layer-2 protocol of the IEEE 802.11 manages, coordinates and maintains communications, traffic, and data distribution in wireless networks that have fixed access points, or in ad hoc networks. The IEEE 802.11 MAC protocol defines beacon frames sent at regular intervals known as beacon intervals, which may be transmitted, for example, every 100 milliseconds by an access point. The beacons allow WLAN devices to monitor for the presence of the access point. Passive and active scanning techniques have been developed for WLAN devices to detect access points, although the 802.11 standard does not mandate particular methods for scanning.
Passive scanning allows the network interface card (NIC) of a WLAN device to find an IEEE 802.11 network by listening for traffic. By listening it is meant monitoring a known channel, and determining if there is a signal present on the channel. As defined in 802.11, passive scanning involves a WLAN device listening to each frequency channel for no longer than a maximum duration defined by the ChannelTime parameter. In this passive mode, the wireless NIC listens for beacons and probe responses while extracting information about the particular channel. Passive scanning expends time and battery power while listening for a beacon frame that may never occur or may be on an idle channel.
The ChannelTime is configured during the initialization stage of the WLAN device driver. To initiate a passive scan, the driver commands the firmware to perform a passive scan with a list of channels. The firmware sequences through the list of channels and sends any received frames to the driver. The amount of time spent on the channel is equal to the ChannelTime value. The driver is able to abort the passive scan when the desired beacon or probe response is received.
Active scanning, in contrast to passive scanning, requires the scanning wireless NIC to transmit a probe request, and receive probe responses from other 802.11 wireless NICs and access points. Active scanning allows the mobile wireless NIC to interact with another wireless NIC or access point based on probe requests and probe responses.
The active scanning of the IEEE 802.11 MAC uses a set of management frames including probe request frames that are sent by a WLAN device and are followed by probe response frames sent by an available access point. In this way, a WLAN device may scan actively to locate an access point operating on a certain channel frequency and the access point can indicate to the WLAN device what parameter settings it is using.
In an active scan, the WLAN device transmits a probe request frame including a service set identifier, and if there is a access point on the same channel that matches the service set identity (SSID) in the probe request frame, the access point will respond by sending a probe response frame to the WLAN device. The probe response includes information the WLAN device uses to access the network. The WLAN device processes the beacon frames and any additional probe responses that it may receive.
Once the various responses are processed or it has been determined that no response has been received within a prescribed time, a WLAN device may continue to scan on another radio channel. At the end of the scanning process, the WLAN device has accumulated data about the networks in its vicinity, and the device can determine which network to join. When compared to passive scanning, active scanning results in longer battery life for the WLAN device, but it also reduces network capacity.
After passive or active scanning, a WLAN device registers itself with an access point (AP) of the chosen network, synchronizes with the AP and, thereafter, transmits and receives data to and from the AP. According to the IEEE 802.11 standard, the registration includes an authentication whereby the AP identifies whether a WLAN device has permission to access the wireless network via a medium access control (MAC) layer. Generally, this authentication phase requires bi-directional authentication steps with the AP and WLAN device exchanging some packets, and optionally, may include additional steps of assertion of identity, challenge of assertion, and response to challenge. After authentication, the WLAN device establishes a connection link with the AP by sending an association request packet to the AP and waiting to receive a response frame from the AP that acknowledges the association. The WLAN device joins a basic service set (BSS) by setting its local hopping time and channel sequence according to the information contained in the AP beacon.
The AP is the timing master of the network, performing a TSF (timing synchronization function) to keep the timers for all WLAN devices synchronized within the same basic service set (BSS) of a larger network. The beacons that are broadcast at fixed time intervals by the AP contain copies of the TSF timer and hopping sequence to synchronize other WLAN devices in a BSS. When a timestamp of a device's TSF timer is different from the timestamp in the received beacon frame, the WLAN device resets its timestamp value to match the received timestamp value.
Providing more reliable and stable communication links for a wireless network depends in part on improving the management of network traffic and decreasing interference among networks devices and other networks. Access points typically execute a hopping pattern, one of the 66 hopping patterns being specified in the IEEE 802.11 draft standard, with hops across non-overlapping frequencies at a rate of, for example, one hop every 100 milliseconds. According to the IEEE 802.11 wireless LAN standard of Frequency Hopping Spread Spectrum (FHSS), the bandwidth used for radio frequency (RF) transmissions is between 2.40 GHz and 2.50 GHz among the 79 channels that are regulated by the Federal Communications Commission (FCC) and used in the U.S. and Canada. Frequency hopping spread-spectrum systems may be selected over direct sequence spread spectrum (DSSS) to minimize interference and increase network capacity.
Other wireless technologies such as Bluetooth for Wireless Personal Area Networks (WPANs) also employ scanning methods. WPAN networks can use spread spectrum techniques that improve transmission reception quality by using fast or slow frequency hopping and direct sequence spread spectrum, the fast frequency hopping changing the frequency more quickly than the modulation rate. A method of communication between access-point devices and mobile devices using the IEEE 802.15 Bluetooth standard is described in “Radio Communication Arrangements,” Melpignano, U.S. patent application No. 2002/0176445 published Nov. 28, 2002. One of the devices enters into a page-scan state where it can receive transmissions on a particular page-scan frequency from another device that transmits a page train. The page train is based in part on an estimate of the page-scan frequency that is determined after communications have occurred between the devices, and the second device has transmitted a page train to the first device. Under predetermined circumstances, the page train is modified and preferably truncated to start on a frequency shifted to correspond to the estimate.
An example of a method by which management functions are transferred among participant devices using Bluetooth page-train procedures is described in “Approach for Managing Communications Channels Based on Performance and Transferring Functions between Participants in a Communications Arrangement,” Treister et al., U.S. patent application No. 2002/0116460 published Aug. 22, 2002. This communication arrangement uses a scan list and timing information to reduce the amount of time spent in acquiring a new management device or access point.
Various methods have been designed to create synchronous and cellular-like communication between devices using Bluetooth and other wireless communication protocols. One proposed method for providing a handoff of sessions between access points while a device is roaming is described in “Wireless Private Branch Exchange (WPBX) and Communicating Between Mobile Units and Base Stations,” Arazi et al., U.S. Pat. No. 6,430,395 issued Aug. 6, 2002. The method synchronizes a mobile device and a switch having small coverage area.
The total time that is consumed for devices using IEEE 802.11 WLAN and other wireless communication technologies to complete all the steps of scanning, authentication and association can vary greatly. Thus, improving the scanning process for wireless networks would help the establishment of a connection between devices and the communication within a network to become more predictable, as well as to become more power and time efficient, particularly for battery-powered IEEE 802.11 WLAN devices. More effective programming techniques for scanning would minimize the number of probe requests generated, the amount of time the receiver of the device is set to an on-state, and the number of times the firmware is interrupting a host controller for beacon processing. Thus, the improved scanning system would increase the battery life of a WLAN device, because the device would need less time to scan or monitor for beacon signals from a primary as well as neighboring access points. In addition, improvements of the scanning system for a WLAN network would benefit associated networks such as wide area networks (WAN), personal area networks (PAN), and controller area networks (CAN). Furthermore, some WLAN frequency bands are shared with certain radar bands. As such, WLAN devices are not allowed to transmit, such as by using active probe scanning. Therefore there exists a need by passive scanning may be performed in a manner that least impacts battery life, and still allows mobile stations to detect the presence of an access point quickly.